Method for inserting nested interbody spinal fusion implants

ABSTRACT

Tapered root threaded hollow perforated interbody spinal fusion implants are disclosed for placement into a disc space in a human spine between adjacent vertebral bodies. The implants have opposite arcuate portions with lockable screws passing therethrough for engaging each of the adjacent vertebral bodies. The implants are adapted for use in side-by-side pairs such that a portion of the circumference of a first implant nests within the circumference of a second implant, so as to have a reduced combined width.

RELATED APPLICATION

[0001] This application is a divisional of U.S. application Ser. No.10/246,931, filed Sep. 19, 2002; which is a divisional of U.S.application Ser. No. 09/566,272, filed May 5, 2000; which claims thebenefit of U.S. Provisional Application No. 60/132,665, filed May 5,1999; all of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION DESCRIPTION OF THE RELATED ART

[0002] The use of hollow threaded perforated interbody spinal fusionimplants such as taught by U.S. Pat. No. 5,015,247 to Michelson ('247),incorporated by reference herein, is now commonplace. Nevertheless,because of the structure and biomechanical properties of these implants,the use of such implants has not been available for all patientsrequiring spinal fusion, but rather has been limited to a subset of thatpopulation. While such implants have proven to be very successful whenused correctly, such success has not been universal.

[0003] A previously identified problem as discussed in U.S. Pat. No.4,593,409 to Michelson ('409), incorporated by reference herein, is thefrequent need for such implants to have a reduced combined widthrelative to their combined height. This permits the height, which isusually the implant diameter, to be sufficiently great so as to span theheight of the distracted disc space and adequately penetrate and engageeach of the vertebral bodies adjacent that disc space, and yet have asignificantly lesser width so that when such implants are utilized inside-by-side pairs, the combined width is such that the paired implantsdo not protrude beyond the width of the spine. Historically, this notinfrequent situation has deprived many patients needing spinal fusionfrom use of the prior art technology as implants of the desired heightcould not safely be placed within that patient's disc space because ofthe width problem. Alternatively, downsized versions of these implantswere implanted with poor results as the implants were of insufficientsize to adequately function for their intended purpose.

[0004] As discussed in Michelson '409, implants having various vertebralbone engaging surface projections have the advantage of enhancedstability within the spine as compared to an implant having a smoothsurface. The use of a thread or thread portions has proven particularlybeneficial and have been described in Michelson '247. As described inMichelson's co-pending application Ser. Nos. 08/484,928; 08/480,904; and08/480,908 incorporated by reference herein, similar devices in whichopposite vertebrae engaging arcuate surfaces are in angular relationshipto each other may be useful to be fuse the vertebrae in a more lordoticangular relationship relative to each other. Implants of the related artare taller near the end adapted to be placed proximate the anterioraspect of the vertebral bodies than at the opposite end adapted to beplaced proximate the posterior aspect of vertebral bodies. The relatedart implants are generally wedge-shaped when viewed from the side. Thewedged configuration causes the implant to be less stable within thespine than if it were non-wedged. Further limiting the stability ofthese implants, compromising the surface area available for contact andfusion, and limiting the volume of osteogenic material containablewithin the implants is a result of the fact that these implants havegenerally been relatively flat across their trailing ends so as to berotationally symmetrical about their mid-longitudinal axes. The anterioraspects of the vertebral bodies are generally curved from side-to-side.As a result, related art implants needed to be rather deeply inset intothe disc space and away from the anterior aspects of the vertebralbodies so as to prevent the implants from protruding from the disc spaceat their lateral wall and trailing end junctions, wheresuch a protrusionof the implant could place vital structures adjacent the spine at risk.

[0005] There is, therefore, a need for further improvement in the designof such interbody spinal fusion implants so as to firstly extend theirrange of usefulness, and secondly to further increase the rate ofsuccess when such implants are used.

SUMMARY OF THE INVENTION

[0006] In accordance with the present invention, as embodied and broadlydescribed herein, there are provided interbody spinal fusion implantsthat are threaded at least in part and require an element of rotationfor insertion across a disc space between two adjacent vertebral bodiesof a spine. The implants of the present invention are configured to bepositioned in close proximity to each other such that the combined widthof the implants is less than the combined height of the implants. Theimplants preferably have a leading end, a trailing end opposite theleading end, and a mid-longitudinal axis and length therebetween. Theimplants preferably have opposite arcuate portions adapted for placementtoward and at least in part within the adjacent vertebral bodies andhave a distance therebetween defining an implant height greater than thenormal height of the disc space to be fused. Each of the oppositearcuate portions preferably has at least one opening in communicationwith each other for permitting for the growth of bone from vertebralbody to adjacent vertebral body through the implant. Preferably, atleast a portion of a thread is formed on the exterior of each of theopposite arcuate portions for penetrably engaging the adjacent vertebralbodies and to facilitate securing the implant into the spine by at leastin part rotating the implant about its mid-longitudinal axis. At least afirst one of the implants preferably has a lateral side wall and amedial side wall with a distance therebetween defining an implant widthtransverse to the implant height. The width of the first implant is lessthan its height along at least a portion of its length. The medial sidewall of the first implant is preferably configured to be positioned inclose proximity to at least a second spinal implant such that thecombined width of the first and second implants is less than thecombined height of those implants.

[0007] The present invention provides for improved interbody spinalfusion implants for placement within the spine in longitudinalside-by-side nested pairs. As used herein, the terms “nesting or nested”refer to the placement of at least two implants in side-by-siderelationship and close proximity to each other. In a preferredembodiment, the present invention teaches the nesting together of a pairof tapered root threaded spinal fusion implants, such that the nestedimplant pair has a combined reduced width relative to the combinedheight of the individual implants. As used herein, the terms “taperedroot” refers to an implant having an outer diameter as measured at thepeaks of the bone penetrating protrusions, such as threads, and a rootdiameter, wherein the root diameter tapers from one end to the other endof the implant.

[0008] An embodiment of the present invention includes an interbodyspinal fusion implant adapted to receive along its length a secondcircumferentially threaded complimentary interbody spinal fusionimplant, such that the second implant nests within the circumference ofthe first implant. The nested longitudinal side-by-side pair has acombined width less than the implants' combined maximum diameters, whichmaximum diameters generally define the over-all implant heights. In afurther embodiment, the implants of the present invention are angledtoward each other such that the combined width at the leading ends isfurther lessened.

[0009] The present invention implants have opposite arcuate portions andpreferably are rotated into place. In an embodiment of the presentinvention, the implants may be generally cylindrical and have a threador thread portions. In a preferred embodiment, the root diameter of theimplant is generally conical or a portion of cone in that the oppositearcuate surfaces for contacting the vertebrae adjacent the disc spaceare in angular relationship to each other generally over the length ofthe implants. In a preferred embodiment of a tapered root implant, theouter diameter of the implant as measured at the thread peaks remainsrelatively constant over the length of most of the implant. As the rootdiameter of the implant tapers down, the thread height increases suchthat the outer diameter of the implant as measured at the thread peaksremains relatively constant.

[0010] The present invention has at least one of a pair of implantshaving at least one side adapted to receive within the over-allcircumference of the outer diameter of the implant the side of a secondimplant. In a preferred embodiment, the receiving implant has bothleading and trailing support walls, and while preferable, but notrequisite, these walls may provide structural support and neverthelessbe highly perforated to allow for vascular access and the growth of bonethrough the implant. In a preferred embodiment of a second implant to bereceived within the first implant, the leading end support structurefurther comprises a cap, which cap need not, but may be threaded, andwhich cap need not be, but preferably is perforated.

[0011] In a preferred embodiment of the present invention, the trailingends of the implants are rotationally asymmetrical about themid-longitudinal axis such that they may be inserted in nested fashionand in proper rotational alignment relative to each other and to thevertebral bodies, with the result that the implants will have a lengthalong the lateral aspect from leading to trailing end less than thelength along the medial wall from leading end to trailing end.Preferably, the implants of the present invention are structurallyadapted, such that when properly inserted, the length of the lateralside wall as measured from the leading end to the trailing end is of alesser length than the length of the implant along its mid-longitudinalaxis so as to prevent the protrusion of the lateral side wall andtrailing end junction beyond the circumferentially curved profile of thevertebral bodies.

[0012] While the present invention does not require it, in a preferredembodiment the implants are adapted to receive through their trailingends opposed bone screws and to transmit at least threaded portions ofthose screws through the opposite vertebrae engaging arcuate portions soas to allow those bone screws to engage at least one each into each ofthe vertebral bodies adjacent a disc space into which the implant areimplanted.

[0013] Each of the embodiments of the implants of the present inventionmay also include one or more of anatomically contoured trailing ends,tapered minor diameters, opposed bone engaging screws, and locks forlocking the opposed bone engaging screws into place. In a preferredembodiment, the implants of the present invention are configured toreceive bone screw locks to lock the opposed bone screws to theimplants. The bone screws are preferably lag screws and the locks, whilepreventing the backing out of the lag screws from the implant, mayeither be rigidly fixed or allow for continuing angular motion of thelag screws relative to the implants.

[0014] The present invention also is directed to an improved method forinserting such implants.

[0015] The accompanying drawings, which are incorporated in andconstitute a part of this specification, are by way of example only andnot limitation, and illustrate several embodiments of the invention,which together with the description, serve to explain the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a side elevational view of the lateral aspect of thehuman spine with implants of an embodiment of the present inventionimplanted therein.

[0017]FIG. 2 is a front perspective view of two adjacent lumbarvertebrae and two implants of an embodiment of the present inventionimplanted across the disc space.

[0018]FIG. 3A is a top plan view of the implants of FIG. 2 and a crosssection through a vertebra along line 3A-3A of FIG. 2.

[0019]FIG. 3B is a top plan view of another embodiment of the implantsof FIG. 3A and a cross section through a vertebra along line 3A-3A ofFIG. 2.

[0020]FIG. 3C is a top plan view of another embodiment of the implantsof FIG. 3A and a cross section through a vertebra along line 3A-3A ofFIG. 2.

[0021]FIG. 4A is a top plan view of the implants of FIG. 3A.

[0022]FIG. 4B is an end view of another embodiment of an implant of thepresent invention with two implants shown in hidden line.

[0023]FIG. 5 is a side perspective view of an embodiment of an implantof the present invention.

[0024]FIG. 6 is a side perspective view of another embodiment of animplant of the present invention.

[0025]FIG. 7 is a side elevational view of the implant of FIG. 5.

[0026]FIG. 8 is a leading end view of the implant of FIG. 6.

[0027]FIG. 9 is a front elevational view of two adjacent vertebrae in alumbar spine with the implants of one embodiment of the presentinvention implanted across the disc space therebetween.

[0028]FIG. 10 is a cross sectional side elevational view of the lateralaspect of two adjacent lumbar vertebrae illustrating the bore created bya drill.

[0029]FIG. 11 is a cross sectional side elevational view of the lateralaspect of two adjacent vertebrae and an embodiment of an implant of thepresent invention inserted therebetween.

[0030]FIG. 12 is a cross sectional side elevational view of the adjacentvertebrae and implant of FIG. 11 with a screw driver and a bone engagingscrew being installed.

[0031]FIG. 13 is a side elevational cross sectional view of the adjacentvertebrae and the implant of FIG. 11 with a bone engaging screw in theinstalled position.

[0032]FIG. 14 is a side elevational cross sectional view of the adjacentvertebrae and implant of FIG. 11 with two bone engaging screws in theinstalled positioned.

DETAILED DESCRIPTION OF THE DRAWINGS

[0033] The following description is intended to be representative onlyand not limiting and many variations can be anticipated according tothese teachings, which are included within the scope of the presentinvention. Reference will now be made in detail to the preferredembodiments of this invention, examples of which are illustrated in theaccompanying drawings.

[0034]FIG. 1 is a side elevational view of the lateral aspect of a humanlumbar spine S having vertebral bodies V and discs D interposedtherebetween. The trailing end of an implant 100 can be seen located inthe disc space between the fourth and fifth lumbar vertebrae L₄ and L₅wherein disc space D has been distracted more anteriorly thanposteriorly.

[0035]FIG. 2 is a front perspective view of adjacent vertebrae within alumbar spine designated as V₁ and V₂. Interposed therebetween is a discD, with paired implants 100 and 200 in nested side-by-side oppositioninserted in the disc space. Implants 100 and 200 have heights greaterthan the height of the restored disc space into which they are implantedso as to expand the height of that disc space and to further penetrablyengage into the bone of each of vertebral bodies V₁, V₂. In a preferredembodiment, the circumference of the trailing end of implant 100preferably extends within the circumference of implant 200 when implants100, 200 are in nested, longitudinal side-by-side placement.

[0036] As shown in FIGS. 3A, 4A, and 5-8, implants 100, 200 preferablyeach have a leading end 108, 208 for insertion first into the discspace; a trailing end 104, 204, opposite leading ends 108, 208,respectively; and a mid-longitudinal axis and length therebetween.Implants 100, 200 preferably each have upper and lower opposite arcuateportions 134, 134′, and 234, 234′ adapted for placement toward and atleast in part within adjacent vertebral bodies V₁, V₂ and have adistance therebetween defining an implant height greater than the normalheight of the disc space to be fused.

[0037] Preferably, opposite arcuate portions 134,134′ and 234, 234′ eachhave at least one opening 120, 220 communicating with one another forpermitting the growth of bone from vertebral body to adjacent vertebralbody through the implant. Preferably, at least a portion of a bonepenetrating protrusion, such as thread 116, 216 is formed on theexterior of each of the opposite arcuate portions 134, 234,respectively, for penetrably engaging the adjacent vertebral bodies andto facilitate securing the implant into the spine by at least in partrotating the implant about its mid-longitudinal axis.

[0038]FIGS. 5-8 show that in a preferred embodiment, implants 100, 200have a body which is generally frusto-conical, increasing in diameterfrom leading end 108, 208 to trailing end 104, 204 and preferably has ahelical thread 116, 216 about its circumference. Thread 116, 216 canhave a generally constant outside diameter and can progress from a sharppointed profile at the thread portion proximate leading end 108, 208 toa thicker and more squared profile toward trailing end 104, 204. Thread116, 216 may be interrupted as shown in FIGS. 5 and 6. These specificsare shown by way of example only and not limitation. It is appreciatedthat implants 100, 200 may have any type of thread or threads useful forthe intended purpose without departing from the present invention.Further, the implants of the present invention need not befrusto-conical as shown, and could be shaped much like a cylinder cut inhalf transversely through its mid-longitudinal axis with the upper andlower halves of the implant in angular relationship to each other.

[0039] In a preferred embodiment, the root diameter of the implants ofthe present invention tapers from its trailing end for placementanteriorly to its leading end for placement posterior within a discspace. This configuration is particularly desirable for providing forthe proper lordotic relationship between the adjacent vertebral bodies,such that those vertebral bodies are distanced apart greater anteriorlythan posteriorly. In a further preferred embodiment of these lordoticimplants, the thread has a generally constant outside diameter such thatin combination with the tapered root the actual thread height increasesfrom the trailing end to the leading end progressively. At the leadingend the thread may have a lesser height for facilitating the implantinsertion without departing from this teaching.

[0040]FIGS. 5 and 7 show a preferred embodiment of implant 200. Lateralside wall 228 and medial side wall 232 have a distance therebetweendefining an implant width transverse to the implant height. The width ofimplant 200 is less than its height along at least a portion of itslength. Medial side wall 232 is preferably configured to be positionedin close proximity to at least implant 100 such that the combined widthof implants 100, 200 is less than the combined height of those implants.

[0041] Implant 200 is similar to implant 100, but differs from implant100 in that while the lateral sides 128 and 228 of implants 100 and 200,respectively, are the same and in this example convex, the medial side232 of implant 200 has been relieved so as to allow for the convexmedial side 132 of implant 100 to protrude therein. Alternatively,medial side of implant 200 can be relieved, in part absent, and/orconcave.

[0042] Implant 200 also has at medial side 232 a convexity as shown bythe contour of trailing support wall 260. In a preferred embodiment,leading support wall 256 may similarly be concave. And further a portionof the medial side wall 232 is absent so as to allow for the protrusionof implant 100 therein.

[0043] As shown in FIG. 4B, in another embodiment of the presentinvention, an implant 300 may have a lateral side wall 328 and anopposite medial side wall 332. Both lateral side wall 328 and medialside wall 332 can be configured to be positioned in close proximity totwo implants 100, one on each side of implant 300. The combined width ofimplant 300 and two implants 100 is less than the combined height ofthose implants.

[0044] Without departing from the scope of the present invention, themedial side of the implant need not be present as a single large opening252 and could have a wall or support portion to it. In a preferredembodiment as shown, the large opening 252 allows for the easy packingof the implant with such osteogenic material as bone or a carriercontaining bone morphogenetic protein or genetic material coding for theproduction of bone. A further advantage of the openness of this area isthat it allows for further collateral vascularization to support bonegrowth from implant to implant. Again, while the present invention isnot limited to a medial opening as shown, implant wall edge 268 can becontinuous longitudinally to provide for strength in this critical areaand if desired can be sharpened as can interrupted thread bases 272 soas to further facilitate the self-tapping nature of these implants.

[0045] In a preferred embodiment of the present invention, leading ends108, 208 and trailing ends 104, 204 are also perforated. While leadingend 208 could be more or less open, in a preferred embodiment there is astructural support portion 256 perforated by openings herein shown as276. Trailing ends 104, 204 preferably have a plurality of openings 144,244 therethrough to allow for vascular access into implants 100, 200 andthe possibility of bone growth therethrough. Openings 144, 244 may alsocommunicate with further openings such as 148, 148′, 248, 248′ tofurther increase the porosity of the trailing end of the implant.Implant 200 preferably is both hollow and highly perforated.Alternatively, the implants of the present invention can comprise aporous type of material such as a cancellously structured tantalum.

[0046] As can be appreciated from FIGS. 3A-4A, implant 100 is preferablystructurally configured so as to cooperatively interdigitate into themaximum circumference of implant 200 along its length. When implant 100is inserted within the circumference of implant 200 as shown in FIGS.3A-4A, the combined width of implants 100 and 200 is substantially lessthan their combined heights, which heights in this case correspond tothe maximum circumference of each of the implants which in this exampleis the same.

[0047] While the implants of the present invention can have any of avariety of configurations at their trailing ends, trailing end 104 ofimplant 100, and trailing end 204 of implant 200 are preferablycontoured to sit on the anterior rim AR of vertebral body V₁. As shownin FIG. 3A, trailing ends 104, 204 are asymmetrical about themid-longitudinal axis of the implants 100, 200, respectively. In thefinal installed position of the implants, trailing ends 104, 204preferably and generally conform to at least a portion of the anatomiccurvature of the anterior rim AR of vertebral bodies V_(1, 2).

[0048] As shown in FIG. 3B, alternatively, trailing end 104′ of implant100′ may be symmetrical about the mid-longitudinal axis of implant 100′and trailing end 204′ of implant 200′ may be asymmetrical about themid-longitudinal axis of implant 200′. Trailing ends 104′, 204′ arepreferably contoured to sit on the anterior rim AR of vertebral body V₂.

[0049] As shown in FIG. 3C, alternatively, trailing end 104″ of implant100″ and trailing end 204″ may be symmetrical about the mid-longitudinalaxes of implants 100″, 200″, respectively. Trailing ends 104″, 204″ arepreferably contoured to sit on the anterior rim AR of vertebral body V₂.

[0050] As best shown in FIG. 8, leading end 108 of implant 100 comprisesan externally threaded cap 112 for threading into the internallythreaded opening of leading end 108. Cap 112 is rotatable in eitherdirection for opening and closing of implant 100 by cap 112 with hexopening 172 which can be manipulated with a hex driver. The specifics ofcap 112, such as the bone holes 176, are shown by way of example onlyand not limitation. Cap 112 is useful for allowing for the compressiveloading of the implant with osteogenic materials such as bone,demineralized bone matrix, carriers such as collagen or any othermaterial useful as a carrier for the delivery of bone producingmaterials such as morphogenetic proteins, mineralizing proteins, orchemical compounds or genetic material coding for the production ofbone. The cap itself may be more or less perforated and the openingthemselves can be either larger or smaller, less or more, as desired toprovide access to the interior of the implant, bone growth therethrough,and to the extent desired acts so as to contain the osteogenic materialgenerally within the implant and from grossly expelling out of theleading end. The cap may be used as a structural member adding strengthto the implant overall. Depending on the specific qualities desired, thecap can be made of any material appropriate for its purpose. Suchmaterials would include various plastics, including polyethylene, andmay include bioresorbable plastics as well. Such materials may includecortical bone, ceramic, or any surgical quality metal suitable for theintended purpose and including by way of example only and notlimitation, surgical grade titanium and its alloys.

[0051]FIG. 4A is a top view of nested implants 100, 200 of FIG. 3A. Asshown in FIG. 4A, the implants of an embodiment of the present inventionare threaded on their exterior surface. While the embodiments shown havebone penetrating projections in the form of a helical thread, thepresent invention allows for a continuous thread, or an implant havingthread portions such as would be useful for an implant to be inserted bylinear advancement and then requiring an element of rotation, forexample 90 degrees, to set the thread portions into the bone of theadjacent vertebral bodies.

[0052] As an alternative, the bone penetrating projecting segments neednot be portions of a thread, but rather could be concentric projectionsgenerally, but not necessarily, oriented perpendicular to the long axisof the implants. For example, the projection may be a ridge or a finadapted to penetrably engage the bone of the adjacent vertebral bodies.

[0053] As shown in FIG. 5, thread 116 of implant 100 may have agenerally constant outer diameter. Inasmuch as the body of implant 100is generally conical such that it tapers from the larger trailing end104 to the smaller leading end 108, the height of thread 116 relative tobody 102 increases from trailing end 104 to leading end 108. Thus, whilethe outer diameter of the threads remains generally constant, the heightof the thread increases from trailing end 104 to leading end 108. Thisis similarly true for implant 200.

[0054] In a preferred embodiment of implant 100, 200 the start of theexternal thread about the perimeter of the implant is precisely indexedsuch that if the surgeon knows the depth of the bore created, he mayselect an implant of the desired length being less than or equal to thedepth of the bore created and by starting the insertion of the implantin a preferred rotational alignment such as the desired final rotationalalignment the implant when threaded in fully will come to rest such thattrailing end 104, 204 will be correctly rotationally aligned so thatscrew receiving holes 236, 236′ will be oriented correctly towards theadjacent vertebral bodies while the profile of trailing ends 104, 204will correspond to the contour of the anterior vertebral body.

[0055] By way of example, for a bore measured to receive a 30 millimetermaximum length implant having a pitch of three millimeters as anexample, the start of the thread at the implant leading end could beindexed such that the implant could be introduced rotationally orientedexactly as desired for the final positioning. Then, by making tencomplete revolutions of three millimeters each the implant wouldassuredly come to rest with trailing wall 204 appropriately oriented andeither be flush with the anterior vertebral cortices, or minimallycounter-sunk to exactly the extent to which the surgeon caused theimplant to enter the bore prior to initiating rotation. As previouslymentioned, trailing end 204 of implant 200 could be rotationallyasymmetrical, but nevertheless be symmetrical from side-to-side, suchthat each of the sides of the implant would be less protuberantposteriorly than a point along the mid-longitudinal axis such that theimplant could be correctly inserted in increments of less than or equalto 180 degrees of rotation.

[0056] While a preferred embodiment of the present invention is directedto a tapered root implant having a constant outer diameter thread, thepresent invention is not so limited. The present invention also wouldinclude implants having bodies that are more or less cylindrical. Thepresent invention also would include other thread designs including athread having a constant height relative to the body and with or withouta constant outer diameter depending on the shape of the body.

[0057] As shown in FIG. 4A, extending through both upper and lowersurfaces 134, 234 and located approximate trailing ends 104, 204 ofimplants 100, 200 are openings 124,124′, and 224, 224′ respectively, fortransmitting the threaded shafts of bone screws for engaging theadjacent vertebral bodies. While not so limited, a preferred embodimentof the present invention is adapted to receive a cancellous lag screw,having a head portion incapable of passing out of the implant so as tobe contained therein. It is further preferred that the implant of thepresent invention are adapted to receive a lock for locking the oppositevertebrae engaging bone screws to the implants. Still further preferredis that the implants of the present invention are adapted to receive thelag screws and screw locks so as to allow the locks to function toprevent the backing out of the screws while, nevertheless allowing forsome angular motion of the screws relative to the implants.

[0058]FIG. 9 shows a front view of an embodiment of the presentinvention with implants 100, 200 properly implanted across the discspace between adjacent vertebral bodies V₁ and V₂. Openings 144, 244 ofimplants 100, 200, respectively, allow for vascular access throughtrailing ends 104, 204 of implants 100, 200 and for bone growththerethrough. Trailing ends 104, 204 have common openings 126, 226 andsituated essentially therein, are threaded openings 148, 248 forreceiving an implant driver. The implant driver has a distal end for acomplimentary fit within common openings 126, 226 and therethrough arotatable threaded member for threading into openings 148, 248,respectively. Openings 126,226 also are adapted to receive a screwdevice to link the implant to other implants, to a staple, or to receivea locking screw to lock bone engaging screws to the implant as disclosedin Michelson U.S. patent application Ser. No. 08/926,334 incorporatedherein by reference. Common opening 126, 226 also may have thereinopposed and divergently angled openings 136, 236 and 136′, 236′ adaptedto receive opposed vertebral bone engaging screws. Bone screw receivingopenings 136, 236 and 136′, 236′ preferably may have circumferentiallyaround them retaining seats 140, 240 and 140′, 240′ adapted to receiveand to block the passage of the heads of screws to be insertedtherethrough. Retaining seats 140,240, and 140′, 240′ may also beflanged.

[0059] As shown in the preferred embodiment of the present invention,trailing ends 104 and 204 of implants 100 and 200, respectively,preferably are rotationally asymmetrical about the longitudinal axes ofthe implants such that the designated medial side of each of theimplants has a length greater than the lateral sides of the sameimplants. Trailing ends 104, 204 preferably are structured to have alesser length along their lateral sides than through themid-longitudinal axis and are preferably contoured so as to sit on theanterior rims of the vertebral bodies without protruding dangerouslytherefrom as set forth in pending Michelson application Ser. No.09/263,266 incorporated herein by reference. In another embodiment ofthe present invention, the trailing ends of the implants can have amaximum length along the mid-longitudinal axis greater than the lengthalong either of the medial and lateral sidewalls so that the bone screwreceiving holes can be oriented towards the adjacent vertebral bodies inhalf rotation increments rather than requiring a full rotation. Whilefor implant 100 this would require no other modification than asdescribed for the trailing end, in regard to implant 200 each of thelateral and medial side walls would have to be relieved, as shown inFIG. 4B for example, to allow for the receipt of the perimeter ofimplant 100 within the maximum perimeter of implant 200.

[0060]FIGS. 10-14 show a series of steps useful for discussing a methodof use of the present invention implants. Methods for inserting spinalimplants are discussed in part in issued and pending patent applicationsto Michelson U.S. Pat. Nos. 5,593,409, 5,741,253, 5,484,437, Ser. Nos.08/396,414, and 08/480,904, incorporated by reference herein. The discspace to be used is preferably, but not necessarily, distracted tooptimal height and the vertebral bodies preferably, but not necessarily,properly aligned. A pair of overlapping bores are then formed across thedisc space with a bone removal device such as a drill having a diametergreater than the height of a distracted disc space such that arc-shapedportions of bone are removed from each of the vertebral bodies adjacentthe disc space to be fused. The overlapping bores are generally orientedfrom anterior to posterior and preferably stop short of the spinalcanal.

[0061] A bone removal device such as a drill or mill that may be conicalcan be utilized to complement the tapered configuration of the implantbody. As shown in FIG. 10, however, in a preferred method a generallycylindrical drill DR or end mill is utilized to create a generallycylindrical bore for receiving the implants. When a pair of generallycylindrical overlapping bores, preferably but not necessarily, having adiameter generally corresponding to that of the root diameter of theimplant proximate the leading end are formed as per FIG. 3A, theimplants will come to be positioned such that the combined width of theimplants at their leading ends will be less than the combined width ofimplants at their trailing end. That is, the implants will be angled intowards each other from anterior to posterior. This has the furtherbenefit of swinging the junction of the lateral side walls and trailingends further inward and away from escaping the anterior vertebralcortex, thereby avoiding protrusion of the lateral side wall to trailingend junctions and allowing for the installation of larger and longerimplants than might otherwise be possible.

[0062] As has been taught by Michelson in the above identifiedapplications and patents incorporated by reference herein, the discspace may be distracted in any number of ways and held distracted duringthe bore formation portion of the procedure. Some of the preferred waysinclude the use of long distractors, short distractors, and extendedouter sleeves having distractor members for placement within the discspace and between the adjacent vertebral bodies as described byMichelson in the above described applications and patents incorporatedby reference herein. Other distractors such as those which attach to thevertebral bodies as by pins or screws might also be useful for thepresent intended purpose.

[0063] While surgery may be performed through a single bore first, in apreferred embodiment both bores are created in overlapping fashion priorto the insertion of the first implant which in this example is implant200. Implant 200 is affixed to an implant driver which driver preferablyengages the implant at trailing wall 204 by interdigitating with implant200 and further binding to implant 200 by a thread such that it ispossible both to rotate implant 200 in either direction and to push orpull simultaneously. While that may be achieved by having a driver whichinterdigitates with any of the openings into or through rear wall 204and having a rotatable portion for threading into threaded opening 248the present invention is not so limited and may include any driveruseful for the intended purpose.

[0064] After implant 200 is fully seated with medial side wall 228oriented immediately toward the disc space, a complementary implant 100is inserted by allowing it to rotate within the maximum circumference ofimplant 200. Pre-tapping the bores formed across the disc space prior tothe insertion of the implants does not deviate from the presentteaching. In a preferred embodiment, pre-tapping is not required ascertain preferred embodiments of the present implants are tapered fromtheir trailing to their leading ends and the leading ends haveparticularly significant thread heights making their ability to threadthemselves into the bone particularly effective.

[0065]FIGS. 11-14, show openings at the trailing end of the implant forreceiving opposed screws that may be oriented from the implant into eachof the adjacent vertebral bodies. These screws enter the implant throughthe trailing end and the threaded shafts of the screws pass throughopenings in the opposite upper and lower vertebral body engagingsurfaces of the implants. Shown in FIG. 11 is a cut away through implant100 of FIG. 9. This is a cross section through the mid-longitudinal axisof implant 100 and the adjacent vertebral bodies. FIG. 12 shows a screwdriver 450 driving a bone screw 500 through common opening 126, bonescrew receiving hole 136′ and out opening 124′ through lower vertebraeengaging surface 134′ into adjacent vertebral body V₂. The presentinvention includes the use of any bone screws for this describedpurpose. In preferred embodiments, structure is provided to block thebone screws from disengaging from the implant or backing out. The screwsmay be rigidly locked to the implant or may be prevented from backingout in a manner that still allows for some relative motion between thescrews and the implant. The latter may be beneficial for anticipatingand allowing for some settling of the vertebral bodies towards the discspace.

[0066] In use, as shown in FIG. 12, the driver 450 is assembled to thescrew 500 thereby compressing the head portion of the screw. The screwis then introduced through the trailing end of the implant and directedinto the body of one of the adjacent vertebrae passing out of an openingadapted for that purpose in one of the opposite vertebrae engagingsurfaces of the implant. The head of the screw 500 is too large to passthrough the opening in the implant, and yet is free to spin against theimplant itself making it possible to lag the screw, or that is to drawthe body of the vertebra to the implant and to generate compressive loadbetween the implant and the vertebral body. As shown in FIG. 13, whenthe screw has been fully seated and the driver removed, the head of thescrew is free to reexpand, thereby locking it to the implant. Asmentioned, the present invention includes the use of any opposedvertebrae engaging bone screws, such that at least one each of saidscrews binds the implant to each of the adjacent vertebral bodies. In apreferred embodiment, the screws are prevented from backing out of theimplant. Screws may be locked directly to the implant such that they arerigidly attached thereto, or may be capable of some movement relative tothe implant so as to allow for variation in screw positioning and/orsettling of the vertebrae, and yet be prevented from backing out.

[0067] While the present invention is shown in a preferred embodiment asboth highly perforate and substantially hollow, the implant couldcomprise a generally porous or cancellous material allowing for thegrowth of bone in continuity from vertebrae to vertebrae through theimplant. The present invention implants can comprise of any materialthat is biocompatible and structurally suitable for construction ofthese interbody spinal fusion implants and consistent with the growth ofbone from vertebral body to vertebral body through the implants. To thatend, materials which would be satisfactory might include implant qualitymetals such as surgical quality titanium and its alloys, cobalt chromeor other metals useful for this purpose, cortical bone and in particularhuman cortical bone such as that which might be obtained from one of thetubular long bones of a human body, ceramics, plastics and compositematerials including those incorporating carbon fibre; and such implantsmay further comprise, contain, be treated with or coated with osteogenicmaterials other than bone for the purpose of achieving spinal fusion.Such materials would include but not be limited to bone morphogeneticprotein, ossification inducing proteins, and genes coding for theproduction of bone. Further, the implants may include at least in partor wholly materials bioabsorbable by the human body, which by way ofexample only may include plastics selected from the lactide, lactonefamily, polylactide, polylactone family, glycolic acid derivatives andso forth.

[0068] The implants of the present invention may be coated with,comprised of, be used in combination with, or have a hollow forcontaining bone growth promoting materials, including but not limitedto, bone morphogenetic proteins, hydroxyapatite, and genes coding forthe production of bone. The implants of the present invention can beformed of a material that intrinsically participates in the growth ofbone from one of adjacent vertebral bodies V to the other of adjacentvertebral bodies V.

[0069] While the specific preferred embodiments the implants of thepresent invention have been described, again the present invention isnot so limited. The present invention includes any interbody spinalfusion implants embodying the present teachings including implantshaving opposite surfaces with a thread, portion of a thread, orgenerally concentric fins used to penetrably engage the substance of thevertebral bodies when the implants are rotated about their longitudinalaxes approximately 90 degrees or more.

[0070] The implants of the present invention are preferably hollow witha plurality of openings through the various surfaces of the implant incommunication with the implant hollow. The present invention includeseither fewer, or more openings so long as each of the opposite vertebraeengaging surfaces of the implant have at least one opening incommunication with the other so as to allow for the growth of bone incontinuity from vertebral body to adjacent vertebral body through theimplant.

[0071] There is disclosed in the above description and the drawingsimplants, which fully and effectively accomplish the objectives of thisinvention. However, it will be apparent that variations andmodifications of the disclosed embodiments may be made without departingfrom the principles of the invention or the scope of the appendedclaims.

What is claimed is:
 1. A method for inserting a plurality of spinalfusion implants across a disc space between two adjacent vertebralbodies of a spine, said plurality of implants comprising oppositethreaded arcuate portions adapted for placement toward and at least inpart within the adjacent vertebral bodies and having a distancetherebetween defining an implant height greater than the normal heightof the disc space to be fused, at least a first of said plurality ofimplants implant having a mid-longitudinal axis and at least a medialside along said mid-longitudinal axis configured for placement in closeproximity to a second of said plurality of implants, said methodcomprising the steps of: forming two partially overlapping bores acrossthe disc space by removing arc shaped portions from each of thevertebral bodies adjacent the disc space; inserting said first implantinto one of said overlapping bores with said medial side being orientedfacing the interior of said disc space; inserting with at least anelement of rotation said second implant having a medial side and anopposite lateral side into a second of said overlapping bores, saidlateral side of said second implant being adjacent and in closeproximity to said medial side of said first implant.
 2. The method ofclaim 1, wherein said forming step includes forming bores having adiameter generally corresponding to the root diameter proximate theleading end of the implants.
 3. The method of claim 1, furthercomprising the step of pre-tapping the bores prior to the steps ofinserting the implants.
 4. The method of claim 1, wherein the insertingsteps include the step of positioning the implants at an angle towardeach other within the overlapping bores such that the combined width ofthe implants at their leading ends will be less than the combined widthof the implants at their trailing ends.
 5. The method of claim 1,wherein the inserting step includes rotating that implant such that whenthreaded fully the implant will come to rest so that the trailing endwill be correctly rotationally aligned so that the profile of thetrailing end will correspond to the contour of the anterior vertebralbody.
 6. A method for inserting a plurality of spinal fusion implantsacross a disc space between two adjacent vertebral bodies of a spine, atleast one of the plurality of implants having the form of a threadedpartial cylinder with a mid-longitudinal axis with a portion of theexternal surface being concave and another of the implants having theform of a threaded cylinder with a mid-longitudinal axis wherein thelargest diameter of the partial cylinder and the diameter of thecomplete cylinder are each larger than the disc space between the twoadjacent vertebral bodies, said method comprising the steps of: formingtwo partially overlapping cylindrical holes across the disc spacebetween the two adjacent vertebral bodies; threading the partiallycylindrical spinal fusion implant having a concave portion into one ofthe overlapping cylindrical holes, the concave portion being orientedsuch that the concave portion will substantially lie on an arc definedby the radius of the second cylindrical hole adjacent; and threading thesecond implant into the second of said overlapping holes with the secondimplant in close proximity to the concave portion of the first implant.7. The method of claim 6, wherein said forming step includes formingbores having a diameter generally corresponding to the root diameterproximate the leading end of the implants.
 8. The method of claim 6,further comprising the step of pre-tapping the bores prior to the stepsof inserting the implants.
 9. The method of claim 6, wherein theinserting steps include the step of positioning the implants at an angletoward each other within the overlapping bores such that the combinedwidth of the implants at their leading ends will be less than thecombined width of the implants at their trailing ends.
 10. The method ofclaim 6, wherein the inserting step includes rotating that implant suchthat when threaded fully the implant will come to rest so that thetrailing end will be correctly rotationally aligned so that the profileof the trailing end will correspond to the contour of the anteriorvertebral body.